I don't know if anyone will have a definitive answer on this but..

When the game says "90 minutes until new crops" or whatnot, and the farmers are just watering; If I have five farmers watering will the crops grow faster than if I had one farmer watering.

I am wondering if I shouldn't be making the farmers do something else while waiting.

The crops are on a fixed schedule that is related to the game speed. Watering them will not make them grow faster.

Excellent. That is good to know. Thank you.

That brings another question:

What if they did not water the crops. While playing the game I used to stop them watering the crops to go fishing, for example. Does it interfere with the crops growth?

No, it won't. As LadyCFII said in her previous post, it's on a fixed schedule. Watering (or not) will not affect the crop growth.

Yup! I made my villagers go do other stuff and the crops were fine!! Very useful to know.

But watering the crops will increase the farmers' skills. If they don't have anything else to do, it could be usefull!

True Laurence. It is just that I usually would rather have tech points coming in faster or a building finished more than I want Master Farmers. The food comes even if they aren't Masters of their trade.